The Lower Palaeozoic Welsh Basin was founded on immature continental crust. During late Precambrian-early Cambrian times, volcanism and sedimentation were influenced by NE-SW-trending faults which defined the NW and SE margins of the basin. During the Cambrian, marine sediments infilled a graben and at the end of the Tremadoc widespread tectonism was associated with an island-arc volcanic episode. In the Ordovician this subduction-related activity was succeeded by mainly tholeiitic volcanism related to back-arc extension, with the locus of arc volcanism sited further N, in the Lake District--Leinster Zone of the Caledonides. In Wales, the Ordovician volcanic activity shifted in time and space. In S Wales volcanism persisted from the middle Arenig through the Llanvirn. In N Wales the volcanism can be broadly divided into dominantly pre-Caradoc activity in southern Snowdonia and an intra-Caradoc episode in central and northern Snowdonia. In eastern Wales, including the Welsh Borderland, and in Ll~,n, both episodes are represented. In all areas faults greatly influenced both volcanism and sedimentation. Intrusive activity was dominated by high-level emplacement of sills. Granite (s.l.) stocks are restricted to central and northern Snowdonia and Ll~n and many were coeval with extrusive volcanism.Volcanism in the basin was essentially bimodal with voluminous eruptions of tholeiitic basalts with ocean-floor affinities, and of rhyolites. Minor volumes of andesite to rhyodacite resulted from low-pressure fractional crystallization of the tholeiitic basalts. Available evidence suggests that the rhyolites resulted mainly from crustal fusion, although in some instances evolution by crystal fractionation from intermediate magma has been proposed. Calc-alkaline assemblages are petrographically distinct, of minor occurence and, contrary to previous conclusions, are relatively insignificant in the characterization of the tectonic environment of the basin.Throughout the basin, volcanism was generally succeeded by deposition of black muds and then turbidite-dominated sequences.

A diagcnctic through anchizone to epizone transition is demonstrated in pelitic rocks of thc Lower Palaeozoic marginal basin of Walcs by cxamination of variations in phyllosilicate mineralogy. illite crystallinity and h,, parameter of white micas. This transition represents a temperature range from -150°C to -400°C and the metamorphism is of a lowpressure facies series type. with a geothermal gradient of -4O"Ckm-'. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.Correlation of pelite data with metabasite assemblages is variable. the most consistent relationship being between epizone crystallinity values and epidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnitepumpellyite assemblages in metabasites and it is suggested that the latter represent nonbuffered, and therefore non-diagnostic, assemblages.

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Mafic phyllosilicates in metabasites affected by low-grade regional metamorphism from Wales and eastern North Greenland show variations in their structure and chemistry. These variations are related to four mineral zones in these metabasites, which are recognized on the presence/absence of various key calc-silicate minerals and also actinolite. Zones 1 and 2 equate with the zeolite facies, zone 3 with the prehnite-pumpellyite facies (or prehnite-actinolite facies in rocks with appropriate bulk rock composition) and zone 4 with the greenschist facies. Whilst variations in Fe/(Fe + Mg) in chlorite correlate closely with Fe/(Fe + Mg) ratios in the whole-rock, other chemical variations are clearly unrelated to whole-rock compositions. Contents of Al" are seen to increase systematically in samples from zone 1 through to zone 4, which relate to an increase in temperature. Calibration of alteration temperatures, calculated using the chlorite geothermometer (based on Al" contents) developed for meta-andesites in the Los Azufres geothermal system (Mexico), against x values (an estimate of the proportion of chlorite to swelling component in the mafic phyllosilicates) shows a decrease in the swelling component in passing from zone 1 to zone 4, i.e. with an increase in temperature. Calculated temperatures compare favourably with published stability estimates for the various key calc-silicates and actinolite. These data indicate that the chlorite geothermometer, although developed for meta-andesites from a hydrothermal system, does show a correlation with temperatures estimated from calc-silicate assemblages in metabasites affected by low-grade metamorphism developed on a regional scale.

Evidence for the early Postglacial use of upland environments in the Mesolithic in various parts Britain has been known for a long time. However, until relatively recently such evidence had been remarkably absent from upland south Wales, which includes some of the highest mountain ranges in southern Britain. In this paper we report on new work at the upland location of Waun Fignen Felen which consists of discrete Early and later Mesolithic artefact scatters on the edge of a former lake basin. In describing this example, we focus on the timing of Mesolithic movements into the interior uplands and examine the relationship between humans and the landscape, particularly in respect to the long distance transport of materials and factors likely to have influenced the choice of site location. Some comparative observations are made on the use and perception of landscapes by ethnographic hunter-gatherers.

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The Altar Stone at Stonehenge is a greenish sandstone thought to be of Late Silurian-Devonian ('Old Red Sandstone') age. It is classed as one of the bluestone lithologies which are considered to be exotic to the Salisbury Plain environ, contrasting with the larger sarsen stones, which are a hard, durable silcrete derived from no more than 30km from Stonehenge. It is well established that most of the bluestones are derived from the Mynydd Preseli, in west Wales. However, no Old Red Sandstone rocks crop out in the Preseli; instead a source in the Lower Old Red Sandstone Cosheston Subgroup at Mill Bay, on the shores of Milford Haven, to the south of the Preseli, has been proposed. More recently, on the basis of detailed petrography, a source for the Altar Stone much further to the east, towards the Wales-England border, has been suggested. Quantitative analyses presented here compare data from proposed 2 Stonehenge Altar Stone debris with samples from the Cosheston Subgroup at Mill Bay in west Wales, as well as with a second sandstone type found at Stonehenge which, on palaeontological evidence has been shown to be Lower Palaeozoic in age. The Altar Stone samples have up to 16.7 modal % calcite while the Lower Palaeozoic and Cosheston Subgroup sandstones have less than 0.25 modal %. The Altar Stone also contains up to 3.8 modal % kaolinite and 0.8 modal % barite, minerals that are absent from the other sandstones. Calcite, kaolinite and barite in the Altar Stone samples all occur between the detrital grains and are all thought to be authigenic minerals, which differs markedly with the Cosheston Subgroup and Lower Palaeozoic sandstones. The Cosheston Subgroup sandstone contrasts with the other two sandstone lithologies in having up to 0.7 modal % detrital garnet (<0.08 in both the other two sandstone types). Further differences between the Altar Stone sandstone and the Cosheston Subgroup sandstone are seen when their contained zircons are examined. Not only do they have differing morphologies (size, shape and quality) but U-Pb age dates for the zircons show contrasting populations; the Cosheston Subgroup sample zircon age population is essentially bimodal, with age maxima at 500 and 1500 Ma whilst the Altar Stone zircon population is more diverse, with ages spanning from 472 to 2475 Ma without maxima.Together, all these data confirm that Mill Bay is not the source of the Altar Stone with the abundance of kaolinite in the Altar Stone sample suggesting a source further east than Milford Haven, towards the Wales-England border. The disassociation of the Altar Stone and Milford Haven fully undermines the hypothesis that the bluestones, including the Altar Stone, were transported from west Wales by sea up the Bristol Channel and adds further credence to a totally land-based route, possibly along a natural routeway leading from west Wales to the Severn estuary and beyond. This route, along the valleys followed today by the A40, may well have been significant in prehistory, raising the possibility that the Altar Stone was a...